The ability to measure, nondestructively and in situ the thickness of growing thin films is very advantageous in many industrial applications. For example, it is important to be able to monitor the thickness of paint being sprayed on cars, trucks or aircraft during production. The costs associated with painting vehicles, particularly in assembly line production is quite significant so that applying too thick a paint layer has serious economic repercussions. Alternatively, if there is too thin a paint layer this may result in the vehicle having to be repainted.
Post production painting of vehicles usually involves applying three distinct layers comprising a primer coating or layer applied directly to the metal substrate, a base coating containing the pigment applied on top of the primer coating and a clear coating applied on top of the base coating. The total thickness of these layers is about 0.05 mm to 0.10 mm with about half of the total thickness being due to the top clear coat. It is preferable that each layer be of a uniform thickness and manufacturers are particularly concerned about controlling the thickness of the base coat; however the base coat is the thinnest layer (about 0.01 mm) which makes it very difficult to control its thickness.
There are several known ways of estimating the average thickness of the paint layers. One is to simply weigh the paint used to cover a certain area and, knowing the mean density of the paint, calculate the average thickness which is generally expressed in units of mg/cm.sup.2, known as the "areal density." Disadvantages of this and similar techniques is it is not an in situ technique, it is very labour intensive and does not give any information about the uniformity of the layers.
Another method and device for measuring paint thickness is disclosed in EP-A-0 380 226. This method relies upon irradiating a coating with x-rays and measuring fluorescent x-rays, Compton peaks and Rayleigh peaks from the spectrum of scattered x-rays. There are several drawbacks to the method and device. The device relies upon use of an expensive LO-AX hyperpure germanium detector. The method only works by adding a non-radioactive label (that fluoresces upon activation by x-rays or gamma rays) of atomic number greater than 20 and the label must be compatible with the radiation source due to correlation between fluorescence efficiency and energy of the x-rays. The method involves measuring the ratio of fluorescent x-rays and Compton rays and the ratio of Compton and Rayleigh yields. The fact that this method of measuring paint thickness is dependent upon adding a label is a major drawback since compatibility of the paint and label material must be taken into account. In addition, it is not practicable or economic to add labels to paints in large scale paint applications such as in painting of automobiles. In order to measure paint thickness with useful precision by using the fluorescent x-rays of additives, it is necessary to add several parts per thousand of label material. Such concentration is difficult to maintain as a homogeneous mixture and it can also degrade the weather resistance of the paint. Therefore it would be very advantageous to provide a method of measuring paint thickness which avoids the need for adding labels to the paint.
At present there is no single, reliable, economic method for accurate, in-situ and nondestructive monitoring of paint thickness as it is being applied to substrates. X-ray backscattering is one method which shows promise as a technique for estimating film thicknesses; however, this technique has severe limitations. A simplified model used in considering backscattering of x-rays from a paint layer on a metal backing is based on two assumptions: 1) the x-rays interact with the paint layer only by the mechanism of Compton scattering and because the total Compton scattering cross-section is almost exactly proportional to the mass, the backscattered x-ray intensity should be proportional to the mass/unit area of the paint layer over a broad range of x-ray energies; and 2) that x-rays penetrating through to the steel backing or substrate are fully attenuated or absorbed in the substrate and not back scattered.
The assumption that the intensity of the backscattered x-rays from the paint is almost exactly proportional to the paint thickness over a broad range of x-ray energies usually holds because the paint layer is so thin and comprised of elements of low atomic number. The model breaks down generally because of the assumption that the metal panel is a perfect absorber over the same broad range of energies. This will be more fully discussed below but this drawback has severely limited the application of x-ray backscattering as a viable in situ technique because the backscattered intensity from the substrate exceeds that from the paint by a large factor unless special precautions are observed.